Fuel cell, method for operating full cell and fuel cell system

ABSTRACT

A fuel cell, a method for operating a fuel cell and a fuel cell system, which ensure no dew condensation for a wet reaction gas in the inlet area of gas channels in plates in a fuel cell stack, are provided. Gas channels  2  and heat medium channels are disposed on one surface and the other surface of one plate  1 , respectively. Gas channels are disposed on the other plate such that they face the gas channels  2  in the plate  1 . A gas inlet header  3  is disposed at the upper part of the gas channel  2  in the plate  1  and a heat medium inlet header is disposed at the upper part of the heat medium channels such that they face the gas inlet header on the other side. Cooling water such as heat medium is supplied from the heat medium supply manifold hole  7  to the heat medium inlet header, thereby warming up the same. The water vapor in the reaction gas (wet fuel gas) is prevented from being condensed in the inlet area of the gas channels  2  by heating up the gas inlet header by the heat conduction.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/725,339, filed on Dec. 2, 2003, claiming priority of Japanese PatentApplication Nos. 2002-350541 and 2002-350546, filed on Dec. 2, 2002, theentire contents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell, which is capable ofpreventing the dew condensation at a reaction gas header in a plate byhumidifying the stacked cells in a fuel cell stack, wherein the heatdischarged from the fuel cell stack is efficiently used for humidifyingthe cells. The present invention also relates to a method for operatingsuch a fuel cell and to a fuel cell system, which are suitable for theoperation of such a fuel cell.

2. Description of the Related Art

In a conventional polymer electrolyte fuel cell, an anode (fuelelectrode) and cathode (air electrode) are deposited respectively on onesurface and the other surface of a solid polymer electrolyte membrane toform a unified element as a cell (membrane electrode assembly), and aunit fuel cell is formed by clamping the cell between both a plate (aseparator) having concave groove-shaped fuel gas channels on the surfacefacing the anode and another plate (another separator) having concavegroove-shaped oxidant gas channels on the surface facing the cathode.Such a plurality of unit fuel cells are stacked and unified into asingle unit by fastening the unit fuel cells together, using a boltpassing through the unit fuel cells and end plates adapted onto bothends thereof. Thus, a fuel cell stack is formed by the unit cells. Inthe operation, a fuel gas (hydrogen gas or reforming gas composed ofmainly hydrogen) is supplied into the fuel gas channels and an oxidantgas (normally air) is supplied into the oxidant gas channels, so that aDC electric power is obtained from the electrochemical reaction whichtakes place via the solid polymer electrolyte membrane.

In such a polymer electrolyte fuel cell, it is required to humidify thesolid polymer electrolyte membrane in order to obtain proper protonconductivity during a period for generating the electric power. In theprior art, therefore, the reaction gas (fuel gas and/or oxidant gas) issupplied into gas channels in the plate, after humidifying the reactiongas with a humidifier, so that the solid polymer electrolyte membrane ismaintained in a moist state. In particular, it is preferable that thesolid polymer electrolyte membrane is humidified with the reaction gasat the dew point equal to the temperature of the membrane or the celltemperature or higher in order to obtain sufficiently high protonconductivity.

Regarding the method for supplying a reaction gas having a dew pointnear the cell temperature, U.S. Pat. No. 5,382,478 discloses a method ofhumidifying the reaction gas, using a heat resulting from a fuel cell inthe state where cooling water for the fuel cell comes into contact withthe reaction gas via a water permeable membrane. Since, however, theevaporation heat significantly increases with the increase of thetemperature, the fuel cell is mostly operated at a temperature of 65° C.to 70° C. A further increase of the temperature in the fuel cellrequires a greater difference between the dew point of the reaction gasand the temperature of the fuel cell.

However, for example, when a reaction gas having a dew point near thecell temperature is supplied to in such a plate A as shown in FIG. 5,water vapor is condensed in a manifold B, and therefore the condensedwater, i.e., the dew clogs the inlet of the gas channel C, therebycausing the flow of the reaction gas to be interrupted. Even when thedew point of the reaction gas is set at a temperature smaller than thecell temperature to some extent in order to avoid the above phenomenon,the dew condensation still takes place in gas channels C in response tothe consumption of the reaction gas, so that the dew clogs the gaschannels C and the supply of the reaction gas is suppressed. As aresult, the reaction gas is not uniformly distributed, and therefore theamount of the reaction gas to be supplied to the electrode becomesinsufficient and further the generation of the electric power is notnormally carried out, thereby causing the performance of the fuel cellto be deteriorated. In particular, the gas flow resistance becomeslarger in the vicinity of curved sections in the gas channels C, so thatthe condensed water is adhered thereto, thereby causing the gas channelsto be clogged. In order to avoid this fact, Japanese Patent PublicationNo. 2761059 discloses a technical measure, in which, for example,S-shaped gas channels are replaced with those in the form of straightline and the condensed water is moved to downstream by supplying thereaction gas from top to bottom in the direction of gravity, and inwhich each water supplying channel is further interposed between theadjacent gas channels to enhance an efficiency in cooling the fuel cellstack. Moreover, as for means for preventing the deterioration of thepower generation performance resulting form the condensed water,Japanese Unexamined Patent Application Publication No. 6-89730 disclosesa technical measure, in which, for example, water absorbing elements aredisposed in the gas channels and/or a dry gas is supplied at the middleportion in the gas channels to remove the condensed water.

In the above-described prior arts, the moisture content in the solidpolymer electrolyte membrane and the temperature at the fuel cell stackcan be maintained within predetermined ranges, so that an excellentresponsibility of transferring to a heavy load for the fuel cell stackcan be obtained and therefore a high output can be obtained in a shorttime, thereby enabling a stable operation to be ensured for such a loadvariation. However, regarding the suppression of dew condensation in thevicinity of the gas channel inlet in the case when the dew point of thereaction gas is increased up to a temperature near the cell temperature,satisfactory results cannot be always obtained. Regarding thecountermeasure for the clogging of the reaction gas channels due to thewater condensed in the gas channels, the above-described prior artsrequire either the insertion of water-absorbing material into the gaschannels of the plate or the mounting of holes and channels forsupplying dry gas in the middle portion of the gas channels. This causesto provide a complicate structure in the fuel cell and to require a lotof work for mounting these components and for manufacturing the fuelcell. It may be stated, therefore, that no sufficient countermeasure isyet introduced into the prior arts.

Regarding a fuel cell system in the prior art, it is necessary todispose a heat source inside the fuel cell system in order to increasethe dew point of the reaction gas near the cell temperature. Moreover,when the dew point of the reaction gas is higher than the temperature ofthe fuel cell stack, the dew condensation takes place without delayafter the reaction gas is supplied to the fuel cell stack. Accordingly,it is necessary to control the relationship between the dew point of thereaction gas and the temperature of the fuel cell stack. The prior artmethod for humidifying the reaction gas, using the heat from the fuelcell, and bringing the cooling water for the fuel cell comes intocontact with the reaction gas via a water permeable membrane, ensuresthat the dew point of the reaction gas is always smaller than thetemperature of the fuel cell stack to some extent. In this case,however, the heat of evaporation strongly increases with the increase ofthe temperature, so that the fuel cell is operated at a temperature of65° C. to 70° C. In addition, almost all the heat of the cooling wateris used for humidification, so that it is difficult to recover the heatfrom the cooling water in the case of a cogeneration application.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apolymer electrolyte fuel cell, which is equipped with plates having asimple structure, and, which is capable of being stably operated in thestate where the dew point of the reaction gas is increased close to thecell temperature.

It is another object of the present invention to provide a method foroperating a polymer electrolyte fuel cell the same, which method issuitable for operating the same.

It is another object of the present invention to provide a fuel cellsystem, which includes a heat source for increasing the dew point of thereaction gas such that it approaches the cell temperature, and whichensures an easy control of temperature and a high efficiency inrecovering the heat from the cooling water in the fuel cell.

Moreover, it is another object to provide a fuel cell system, whichincludes polymer electrolyte fuel cells capable of being operated in thestate where the reaction gas has a dew point having an increased amountnear the cell temperature.

To attain the above-mentioned objects, the following measures areprovided in the present invention.

In a first aspect of the invention, a fuel cell is formed by stacking aplurality of plates each having reaction gas channels or heat mediumchannels in a fuel cell stack, wherein an inlet header for at least oneside of reaction gas channels is disposed so as to face the inlet headeror the outlet header for said heat medium channels. In accordance withthe first aspect of the invention, the temperature at the reaction gasinlet header can be maintained by using the heat medium. As a result,water vapor contained in the reaction gas is not condensed and thereforeno dew condensation is generated in the inlet area for the gas channels,thereby enabling a normal operation to be ensured for the generation ofelectric power along with an enhanced performance of the cell.

In a second aspect of the invention, it is included in the first aspectof the invention that the inlet header for said reaction gas channels ismaintained at a temperature of the dew point of gas or greater with aidof the heat medium. In accordance with the second aspect of theinvention, the dew condensation can be securely prevented at thereaction gas inlet header.

In a third aspect of the invention, a fuel cell is formed by stacking aplurality of plates each having reaction gas channels in a fuel cellstack, wherein the inlet header for one side of reaction gas channels isdisposed such that it faces the inlet header for the other side ofreaction gas channels. In accordance with the third aspect of theinvention, the one reaction gas inlet header can be maintained at apredetermined temperature by the other side of reaction gas.

In a fourth aspect of the invention, it is included in the third aspectof the invention that the dew point of the one side of reaction gas isset at the temperature of the other side of reaction gas or smaller inthe fuel cell according to the third aspect. In accordance with thefourth aspect, the inlet header for the one side of reaction gas can bemaintained at a temperature of the dew point or greater by the otherside of reaction gas to prevent dew condensation.

In a fifth aspect of the invention, a fuel cell is formed by stacking aplurality of plates each having reaction gas channels or heat mediumchannels, wherein an inlet header for one side of reaction gas and aninlet header for the other side of reaction gas are disposed such thatthey face an inlet header or an outlet header for the heat medium. Inaccordance with the fifth aspect, the reaction gas inlet header can bemaintained at a predetermined temperature by the heat medium.

In a sixth aspect of the invention, it is included in the fifth aspectof the invention that the one side of reaction gas and the other side ofreaction gas flow parallel to each other from top to bottom in thedirection of gravity, and wherein both sides of reaction gas flow indirection parallel (co-flow) or anti-parallel (counter-flow) to the heatmedium. In accordance with the sixth aspect, the reaction gas can beefficiently heated up by the heat medium.

In a seventh aspect of the invention, it is included in the fifth orsixth aspect of the invention that channels for the one side of gas, theother side of gas and the heat medium are shaped straight in therespective portions facing an anode or cathode electrode section. Inaccordance with the seven aspect of the invention, the straight channelshaving no curved portion ensure a smooth flow of the reaction gas, alongwith a smooth discharge of the condensed water.

In an eighth aspect of the invention, it is included in one of the fifthto seventh aspects of the invention that the dew point of at least oneof the supplied reaction gases ≦the temperature of heat medium at theinlet in the case when at least one of the reaction gases and the heatmedium flow in the direction parallel to each other, whereas the dewpoint of at lease one of the supplied reaction gases ≦the temperature ofthe heat medium at the outlet in the case when at least one of thereaction gases and the heat medium flow in the direction anti-parallelto each other. In accordance with the eight aspect of the invention, thedew condensation can be prevented at the reaction gas inlet header.

In a ninth aspect of the invention, it is included in the eight aspectof the invention that wherein a following equation is established in thecase when at least one of the reaction gases and the heat medium flow inthe direction parallel to each other,

the dew point for at least one side of discharged reaction gas ≧thetemperature of the heat medium at the outlet,whereas another equation is established in the case when at least one ofthe reaction gases and the heat medium flow in the directionanti-parallel to each other,the dew point for at least one side of discharged reaction gas ≧thetemperature of the heat medium at the inlet. In accordance with theninth aspect of the invention, the electrode portions can be humidifiedby condensing the water in the gas channels. Moreover, the variation inthe pressure loss can be reduced by dew condensation in all the gaschannels, thereby enabling the gas distribution to be homogenized. Forinstance, an increased gas flow rate in part of the gas channels causesthe amount of condensed water to be increased. This causes the pressureloss to be increased and further the gas flow rate to be lowered.

In a tenth aspect of the invention, it is included in the third aspectof the invention that the heat medium is supplied so as to flow at anarea facing the downstream area for the reaction gas inlet header,wherein the heat medium heat-exchanged at an area facing the electrodesection is supplied so as to flow at an area facing the reaction gasinlet header. In accordance with the tenth aspect of the invention, thecell can be maintained in a humidified state by dew condensation in thereaction gas at the area facing the downstream area for the inlet headerfor the reaction gas, and at the same time, the dew condensation can beprevented by heating up the reaction gas at the inlet header for thereaction gas.

In an eleventh aspect of the invention, it is included in the tenthaspect of the invention that a following equation is established,

the dew point for at least one side of reaction gas ≧the temperature ofthe heat medium at the inlet. In accordance with the eleventh aspect ofthe invention, the effect resulting from the tenth aspect can beensured.

In a twelfth aspect of the invention, it is included in one of the firstto eleventh aspects of the invention that a flow resistance generationsection is disposed at the inlet for at least one side of reaction gas.In accordance with the twelfth aspect of the invention, the flow of thereaction gas can be regulated, and the distribution of reaction gas inthe respective gas channels can be homogenized, so that the water dewcondensed in the lower level area can be discharged to the outlet of thegas channels by the pushing force.

In a thirteenth aspect of the invention, it is included in the twelfthaspect of the invention that the reaction gas inlet header includes theflow resistance generation section. In accordance with the thirteenthaspect of the invention, the effect resulting from the twelfth aspectcan be ensured. Since, moreover, the flow resistance generation sectioncan be maintained at a temperature equal to or greater than the dewpoint of the reaction gas, the dew condensation can be prevented there.

In a fourteenth aspect of the invention, it is included in any one ofthe first to thirteenth aspects of the invention that an oxidanthumidifier and a fuel humidifier are connected to the fuel cellaccording to any one of the first to thirteenth aspects, and the heatmedium discharged from the fuel cell is heat-exchanged in thesehumidifiers. In accordance with the fourteenth aspect of the invention,a dew point of the reaction gas near the temperature of the cell can beobtained only by the heat in the heat medium in the case of a lowoperation temperature (for example, 70° C. or less).

In a fifteenth aspect of the invention, an oxidant humidifier and/or afuel humidifier, and a total heat exchanger are connected to a fuelcell, and the heat medium discharged from the fuel cell isheat-exchanged in these humidifiers, wherein the total heat exchange iscarried out between at least one side of reaction gas in the reactiongas discharged from the fuel cell and at least one side of reaction gasin the reaction gas before supplied to said humidifiers. In accordancewith the fifteenth aspect of the invention, a humidifying temperaturenear the temperature of the cell can be obtained by the heat in the heatmedium and by using the heat recovered from the gas discharged from thefuel cell, in the case of a high cell temperature (for example, 70° C.or more).

In a sixteenth aspect of the invention, the fuel cell and the method ofoperating the same employed in the fuel cell system according to thefifteenth aspect of the invention are characterized respectively by afuel cell and a method for operating the same according to any one ofthe first to fifteenth aspects of the invention. In accordance with thesixteenth aspect, the effect resulting from the first to fifteenthaspects can be ensured.

In a seventeenth aspect of the invention, it is included in any one ofthe fourteenth to sixteenth aspects of the invention that the heatmedium discharged from the fuel cell is first heat-exchanged in onehumidifier in which either the oxidant gas or fuel gas flows at a higherflow rate, and then heat-exchanged in the other humidifier. Inaccordance with the seventeenth aspect of the invention, when the air isused as an oxidant gas, the heat exchange is firstly carried out in theair humidifier due to a greater flow rate of air. Accordingly, thedifference between the dew point of the oxidant gas and that of the fuelgas can be reduced, thereby enabling the performance of the cell to beenhanced.

As described above, the present invention is usefully applicable to botha polymer electrolyte fuel cell and a method for operating the same. Thepresent invention is also applicable to a fuel cell system, which isused in a power generation system with such a polymer electrolyte fuelcell, a cogeneration system and others.

Further objects, features and advantages of the present invention willbecome apparent form the following description of the preferredembodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a plan view of a first plate (plate 1) mounted in a fuelcell stack according to an embodiment of the present invention viewedfrom the side of the fuel gas channels;

FIG. 1( b) is a plan view of a first plate (plate 1) mounted in a fuelcell stack viewed from the side of the water channels;

FIG. 1( c) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side of the air channels;

FIG. 1( d) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side on which no gas channels areformed;

FIG. 2( a) is a plan view of a first plate (plate 1) mounted in a fuelcell stack according to an embodiment of the present invention viewedfrom the side of the fuel gas channels;

FIG. 2( b) is a plan view of a first plate (plate 1) mounted in a fuelcell stack viewed from the side of the water channels;

FIG. 2( c) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side of the air channels;

FIG. 2( d) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side on which no gas channels areformed;

FIG. 3( a) is a plan view of a first plate (plate 1) mounted in a fuelcell stack according, to an embodiment of the present invention viewedfrom the side of the fuel gas channels;

FIG. 3( b) is a plan view of a first plate (plate 1) mounted in a fuelcell stack viewed from the side of the water channels;

FIG. 3( c) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side of the air channels;

FIG. 3( d) is a plan view of a second plate (the other plate) mounted ina fuel cell stack viewed from the side on which no gas channels areformed;

FIG. 4( a) is a plan view of an embodiment of a flow resistancegeneration section mounted in a fuel cell according to the invention;

FIG. 4( b) is a front view of the same;

FIG. 5 is a plan view of a plate used in a conventional fuel cell;

FIG. 6 is a schematic sectional view of a component in a fuel cell stackused in a fuel cell system according to the invention;

FIG. 7( a) is a plan view of a bipolar plate mounted in a fuel cellstack according to the invention viewed from the side of the fuel gaschannels;

FIG. 7( b) is a plan view of the same viewed from the oxidant gaschannel side;

FIG. 8( a) is a plan view of an anode cooling plate mounted in a fuelcell stack according to invention viewed from the side of the fuel gaschannels;

FIG. 8( b) is a plan view of the same viewed from the side of the waterchannels;

FIG. 9( a) is a plan view of a cathode cooling plate mounted in a fuelcell stack according to the invention viewed from the side of theoxidant gas channels;

FIG. 9( b) is a plan view of the same on the side on which no gaschannels are formed; and

FIG. 10 is a block diagram of a fuel cell system in an embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, several embodiments of afuel cell according to the invention will be described.

Embodiment 1

Each of FIG. 1( a), FIG. 1( b), FIG. 1( c) and FIG. 1( d) is a plan viewof a fuel cell according to the invention in a first embodiment. FIG. 1(a) is a plan view of a first plate mounted in a fuel cell stack, viewedfrom the side of the fuel gas channels and shows the side of a firstsurface of the first plate. FIG. 1( b) is a plan view of the first platemounted in the fuel cell stack, viewed from the side of the waterchannels and shows the side of a second surface of the first plate. FIG.1 (c) is a plan view of the other plate (a second plate) mounted in thefuel cell stack, viewed from the side of the air channels and shows theside of a third surface of the second plate. FIG. 1( d) is a plan viewof the second plate mounted in the fuel cell stack, viewed from the sideon which no gas channels are formed and shows the side of a fourthsurface of the second plate. Reference numeral 1 shown in FIG. 1( a) andFIG. 1( b) and reference numeral 1′ shown in FIG. 1( c) and FIG. 1( d)mean the plates (i.e. the first and second plates) typically made ofcarbon, respectively. In FIG. 1( a), a plurality of concavegroove-shaped gas channels 2 is formed on one surface (the firstsurface) of the plate 1 so as to align in the up/down direction (thedirection of gravity). In FIG. 1( b), a plurality of concavegroove-shaped water channels 7B is formed on the other surface (thesecond surface) of the plate 1 so as to align in the up/down direction(the direction of gravity) to be back to back each other. In this case,a gas supply manifold hold 3 is disposed on the upper left side of theplate 1 so as to pass therethrough, and the gas supply manifold hole 3is connected to a gas inlet header 4 in the concave form. Moreover, thegas inlet header 4 is further connected to the gas channels 2. The gasinlet header 4 is defined as an area where a reaction gas (fuel gas oroxidant gas) supplied in a distributed state from the gas supplymanifold hole 3 is further supplied to the inlet of the gas channels 2(the same definition is applied in the following). Such a gas inletheader 4 is generally called as a manifold.

The outlet of the gas channels 2 is connected to a gas outlet header 5in the concave form, which is disposed in the lower part of the plate 1,and the gas outlet header 5 is connected to a gas discharge manifoldhole 6 which is disposed on the lower left side of the plate 1 so as topass therethrough. As a result, a reaction gas (in this case, fuel gas)is supplied and distributed to the gas inlet header 4 of the plate 1 ineach cell via the gas supply manifold hole 3, which is aligned in thestacking direction of the fuel cell stacks, and then distributed fromthe gas inlet header 4 to the gas channels 2, so that the reaction gasis supplied from top to bottom in the gas channels 2, and thendischarged from the gas outlet header 5. At the same time, the suppliedreaction gas flows into a gas discharge manifold hole 6 aligned in thestacking direction of the fuel cell stack, and finally discharged to theoutside from the end portion of the fuel cell stack via the gasdischarge manifold hole 6.

On the upper right side of the plate 1 (the side opposite to the gassupply manifold hole 3), a water supply manifold hole 7 is disposed soas to pass through the plate 1, and referring to FIG. 1( b) the watersupply manifold hole 7 is connected to a water inlet header 7A in theconcave form, which is disposed on the other surface (second surface) ofthe plate 1. In FIG. 1( b) the water inlet header 7A is furtherconnected to the inlet of the water channels 7B. In this case, the gasinlet header 4 and the water inlet header 7A are disposed respectivelyon the one surface (the first surface) and the other surface (the secondsurface) of the plate 1 to be back to back with each other.

Moreover, a water outlet header 7C in the concave form is disposed atthe outlet of the water channels 7B in the plate 1, and the water outletheader 7C is connected to a water discharge manifold hole 8, which isdisposed on the lower right side of the plate 1 (on the side opposite tothe gas discharge manifold hole 6) so as to pass therethrough. As aresult, the water supplied from the end portion of the fuel cell stack(in this case, cooling water) is supplied and distributed to the waterinlet header 7A of the plate 1 in the respective cells through the watersupply manifold 7, which is aligned in the stacking direction of thefuel cell stack. The supplied water is distributed in water channels 7Bfrom the water inlet header 7A and is supplied from top to bottom alongthe water channels 7B, and then discharged into the water outlet header7C, and further flows into the water discharge manifold hole 8, which isaligned in the stacking direction of the fuel cell stack. Finally, thewater is discharged to the outside from the end portion of the fuel cellstack after passing through the water discharge manifold hole 8.

On the other hand, in FIG. 1( c), a plurality of gas channels 2′corresponding to the gas channels 2 in the plate 1 shown in FIG. 1( a)and FIG. 1( b) is disposed from top to bottom (in the direction ofgravity) in the other plate (the second plate) 1′. A gas inlet header 4′in the concave form is connected to the inlet of the gas channels 2′ anda gas outlet header 5′ in the concave form is connected to the outlet ofthe gas channels 2′. As a result, in the other plate 1′, an oxidant gas(In this case, air introduced from the outside air) is supplied to thegas inlet header 4′, and distributed into the gas channels 2′ from thegas inlet header 4′. Then, the oxidant gas flows from top to bottomalong the gas channels 2′, and discharged to the gas outlet header 5′,and finally discharged to the outside of the fuel cell stack.

A cell is inserted between the gas channels 2 of the plate 1 and the gaschannels 2′ of the other plate 1′, and the composite member thus formedis mounted in the fuel cell stack. In this case, a unit cell isconstituted by contacting closely and facing the anode of the cell tothe gas channels 2 of the plate 1 and by contacting closely and facingthe cathode of the cell to the gas channels 2′ of the other plate 1′.Then, the fuel cell stack is constituted by stacking the unit cells toform a unit cell. Regarding the gas inlet headers 4, 4′, the gas outletheader 5, 5′, the water inlet header 7A and the water outlet header 7C(7C is shown in FIG. 1( b)), the upper surface of the concave portion iscovered by a gasket or the like, thereby enabling the leakage of gas andwater to be prevented.

In the fuel cell thus constituted, the fuel gas flows in the gaschannels 2 of the plate 1 and the oxidant gas flows in the gas channels2′ of the other plate 1′. As a result, an electrochemical reaction takesplace via the polymer electrolyte membrane of the cell, thereby enablingthe DC electric power to be generated.

In order to humidify the polymer electrolytic membrane of the cell inthe saturated state, the fuel gas is supplied to the fuel cell stack,after it is humidified with, for example, a humidifier at a dew pointclose to the cell temperature. In the prior art, a wet fuel gas iscooled particularly in the inlet area of the gas channels 2, when it issupplied to the gas channels 2, so that the water vapor contained in thegas is dew condensed to form the dew. As a result, the condensed wateris adhered to inside wall of the gas channels 2 and clogs them, therebycausing the flow of the fuel gas to be interrupted. In this embodiment,however, the water inlet header 7A is disposed so as to be close to thegas inlet header 4 so as to be back to back with each other. As aresult, the water inlet header 7A is heated by the cooling water as aheat medium supplied thereto, and the gas inlet header 4 is indirectlyheated up by the heat conduction, thereby making it possible to preventthe water vapor contained in the fuel gas from dew condensation.

In order to suppress the dew condensation by the cooling water as a heatmedium, the temperature of the cooling water should be set at the dewpoint of the fuel gas or greater (the dew point of the fuel gas ≦thetemperature of the cooling water at the inlet). Furthermore, it ispreferable that the temperature of the oxidant gas (air) is set so as tofulfill the following relationship: The dew point of the fuel gas ≦thedew point of the air ≦the temperature of the cooling water at the inlet.

In this embodiment, the cooling water is used as a heat medium forpreventing the dew condensation in the fuel gas. However, the oxidantgas can be used for the heat medium instead of the cooling water. Inthis case, the gas inlet header 4′ for the oxidant gas is disposed closeto the gas inlet header 4 for the fuel gas in the plate 1 on the othersurface, although the arrangement is not shown, and water channels 7Bfor supplying the cooling water are disposed in the other plate 1′.Furthermore, in order to prevent the dew condensation of the fuel gas bythe oxidant gas (air), the temperature of the air inlet is set such thatthe dew point of the fuel gas ≦the temperature of the air.

Embodiment 2

Each of FIG. 2( a), FIG. 2( b), FIG. 2( c) and FIG. 2( d) is a plan viewof a fuel cell according to the invention in a second embodiment. Thisembodiment is different from the first embodiment as for the point thatan inner air manifold system is employed in the second embodiment. FIG.2( a) is a plan view of a first plate mounted in a fuel cell stack,viewed from the side of the fuel gas channels and shows the side of afirst surface of the first plate, FIG. 2( b) is a plan view of the firstplate mounted in the fuel cell stack, viewed from the side of the waterchannels and shows the side of a second surface of the first plate. FIG.2( c) is a plan view of the other plate (a second plate) mounted in thefuel cell stack, viewed from the side of the air channels and shows theside of a third surface of the second plate. FIG. 2( d) is a plan viewof the second plate mounted in the fuel cell stack, viewed from the sideon which no gas channels are formed and shows the side of a fourthsurface of the second plate. Reference numeral 1 shown in FIG. 2( a) andFIG. 2( b) and reference numeral 1′ shown in FIG. 2( c) and FIG. 2( d)mean plates (i.e. the first and second plates) typically made of mainlycarbon, respectively. In FIG. 2( a), a plurality of gas channels 2 inthe form of concave grooves are disposed from top to bottom (in thedirection of gravity) on one surface (a first surface) of the plate 1,and a gas supply manifold hole 3 is disposed on the upper left side ofthe plate 1 in such a way that it passes through the plate 1, and thegas supply manifold hole 3 is connected to a gas inlet header 4 in theconcave form. Moreover, the gas inlet header 4 is connected to the gaschannels 2. Such a gas inlet header 4 is generally called as a manifold.In FIG. 2( b), a plurality of water channels 7B in the form of concavegrooves are disposed from top to bottom (in the direction of gravity) onthe other surface (the second surface) of the plate 1 in such a way thatthe gas channels 2 and the water channels 7B are back to back eachother.

Referring to FIG. 2( a) the outlet of the gas channels 2 is connected toa gas outlet header 5 in the form of a concave shape, and the gas outletheader 5 is connected to a gas discharging manifold hole 6, which isdisposed on the lower left side of the plate 1 so as pass therethrough.As a result, a reaction gas (fuel gas) supplied from the end portion ofthe fuel cell stack is distributed to the gas inlet header 4 of theplate 1 in each cell via the gas supply manifold hole 3 which is alignedin the stacking direction of the fuel cell stack, and the reaction gasis further distributed from the gas inlet header 4 into the gas channels2, so that the gas thus distributed flows from top to bottom along thegas channels 2, and it is discharged to the gas outlet header 5 andfurther flows into the gas discharge manifold hole 6 aligned in thestacking direction of the fuel cell stack. Finally, the reaction gas isdischarged from the end portion of the fuel cell stack to the outsidevia the gas discharge manifold hole 6.

In addition, a water supply manifold hole 7 is disposed on the upperright side of the plate 1 (on the side opposite to the gas supplymanifold hole 3). In FIG. 2( b), the water supply manifold hole 7 isconnected to the water inlet header 7A in the form of a concave shape,which is disposed on the other surface of the plate 1, and the waterinlet header 7A is connected to the inlet of the water channels 7B.

Moreover, a water outlet header 7C in the concave form is disposed inthe outlet of the channels on the other surface of the plate 1, and thewater outlet header 7C is connected to a water discharge manifold hole8, which is disposed on the lower right side of the plate 1 (on the sideopposite to the gas discharge manifold hole 6) so as pass therethrough.As a result, water (cooling water), which is supplied from the endportion of the fuel cell stack, is supplied and distributed to the waterinlet header 7A, in the plate 1 of each cell via the water supplymanifold hole 7 aligned in the stacking direction of the fuel cellstack, and the water is further distributed from the water inlet header7A to the water channels 7B. Thereafter, the water flows from top tobottom along the water channels 7B, and it is discharged to the wateroutlet header 7C and then flows into the water discharge manifold hole 8aligned in the stacking direction of the fuel cell stack. Finally, thewater is discharged to the outside from the end portion of the fuel cellstack via the water discharge manifold hole 8.

On the other hand, referring to FIG. 2( c), a plurality of gas channels2′ corresponding to the gas channels 2 in the plate 1 shown in FIG. 2(a) and FIG. 2( b) are arranged from top to bottom (in the direction ofgravity) in the other plate (the second plate) 1′. In this case, a gassupply manifold hole 3′ is disposed on the upper right side of the otherplate 1′ so as to pass therethrough, and the gas supply manifold hole 3′is connected to a gas inlet header 4′ in the concave form, and furtherthe gas inlet header 4′ is connected to the gas channels 2′.

In FIG. 2( c), the outlet of the gas channels 2′ in the other plate 1′is connected to the gas outlet header 5′ in the concave form, which isdisposed in the lower part of the other plate 1′, and the gas outletheader 5′ is further connected to a gas discharge manifold hole 6′,which is disposed in the lower end of the other plate 1′ so as to passtherethrough. As a result, oxidant gas (air) supplied from the endportions of the fuel cell stack is supplied and distributed to the gasinlet header 4′ on the other plate 1′ in each cell via the gas supplymanifold hole 3′ aligned in the stacking direction of the fuel cellstack, and then distributed to the gas channels 2′ from the gas inletheader 4′. The oxidant gas thus distributed flows from top to bottomalong the gas channels 2′, and it is discharged to the gas outlet header5′. Thereafter, the oxidant gas flows into the gas discharge manifoldhole 6′ aligned in the stacking direction of the fuel cell stack and isdischarged from the end portion of the fuel cell stack to the outsidevia the gas discharge manifold hole 6′.

Similarly to the first embodiment, each cell is inserted between the gaschannels 2 in the plate 1 and the gas channels 2′ in the other plate 1′,and the composite elements obtained after the insertion are mounted inthe fuel cell stack. In this case, an anode in the cell faces the gaschannels 2 in the plate 1 and contacts closely thereto, and a cathode inthe cell faces the gas channels 2′ in the other plate 1′ and contactsclosely thereto, so that a unit cell is formed. The fuel cell stack isproduced by stacking such unit cells to form a unit. In this case, thegas inlet header 4, gas outlet header 5, water inlet headers 7A and thewater outlet header 7C are covered on their concave upper surface by agasket or the like, so that the leakage is prevented.

In the fuel cell stack thus formed in the second embodiment, the fuelgas flows into the gas channels 2 of the plate 1 and the oxidant gasflows into the gas channels 2′ of the other plate 1′, so that theelectrochemical reaction takes place via the polymer electrolytemembrane of the cell, thereby enabling a DC electric power to begenerated.

As described above, in order to humidify the polymer electrolytemembrane of the cell in the saturated state, a fuel gas is supplied tothe fuel cell stack after humidified with, for example, a humidifiersuch that the dew point is close to the cell temperature. In the secondembodiment, the water inlet header 7A is disposed such that it is closeto the gas inlet header 4 on the other surface, so that the water inletheader 7A is heated by the cooling water supplied thereto, and the gasinlet header 4 is indirectly heated by the heat conduction, therebymaking it possible to prevent the water vapor contained in the fuel gasfrom condensing. Accordingly, the clogging of the fuel gas due to thecondensed water can be suppressed, and a normal operation in thegeneration of the electric power is ensured, thereby enabling highperformance of the cell to be maintained.

In order to suppress the dew condensation resulting from the coolingwater, the inlet for the cooling water is set at the dew point of thefuel gas or grater (the dew point of the fuel gas ≦the temperature ofthe cooling water at the inlet). Furthermore, it is preferable that thetemperature relationship for the oxidant gas (air) is set such that thedew point of the fuel gas ≦the dew point of the air ≦the temperature ofthe cooling water at the inlet. If, moreover, the cooling water at theoutlet is set at a temperature equal to or smaller than the dew point ofthe reaction gas at the outlet, the solid polymer electrolyte membranecan be securely humidified. In this case, since the dew condensationoccurs in all the gas channels, the deviation in the pressure loss isreduced and therefore a uniform gas distribution can be obtained.

In the second embodiment, the cooling water is also used as a heatmedium for preventing the dew condensation of the fuel gas. However, theoxidant gas can be used as the heat medium instead of the cooling water.In this case, the gas inlet header 4′ for the oxidant gas is disposedclose to the gas inlet header 4 for the fuel gas in the plate 1 on theother surface (the second surface), although the arrangement is notshown, and water channels 7B for supplying the cooling water aredisposed in the other plate 1′. Furthermore, in order to prevent the dewcondensation of the fuel gas by the oxidant gas (air), the temperatureof the air inlet is set such that the dew point of the fuel gas ≦thetemperature of the air.

In the first and second embodiments, the reaction gas and the coolingwater flow in the direction parallel to each other as well as in thedirection of gravity at the area facing the electrodes. However, it ispossible to employ the structural arrangement in which the reaction gasand the cooling water flow in the direction anti-parallel to each other.In this case, the relationship, the dew point of the fuel gas ≦the dewpoint of the air ≦the temperature of the cooling water at the outlet, ispreferably set.

Embodiment 3

Each of FIG. 3( a), FIG. 3( b), FIG. 3( c) and FIG. 3( d) is a plan viewof a fuel cell according to the invention in a third embodiment. FIG. 3(a) is a plan view of a first plate mounted in a fuel cell stack, viewedfrom the side of the fuel gas channels and shows the side of a firstsurface of the first plate. FIG. 3( b) is a plan view of the first platemounted in the fuel cell stack, viewed from the side of the waterchannels and shows the side of a second surface of the first plate. FIG.3( c) is a plan view of the other plate (a second plate) mounted in thefuel cell stack, viewed from the side of the air channels and shows theside of a third surface of the second plate. FIG. 3( d) is a plan viewof the second plate mounted in the fuel cell stack, viewed from the sideon which no gas channels are formed and shows the side of a fourthsurface of the second plate. In FIG. 3( a) and FIG. 3( b), the structureof a plate (the first plate) 1 is basically similar to that in thesecond embodiment. Also, in FIG. 3( c) and FIG. 3( d), the structure ofthe other plate (the second plate) 1′ is basically similar to that inthe second embodiment. As a result, the same reference numeral isattached to the same structural element as in the second embodiment andtherefore detailed description thereof is omitted. Hence, detaileddescription is given exclusively to the structural elements differentfrom those in the second embodiment. A main difference between thesecond and third embodiments is that a flow resistance generationsection 9 is disposed at the inlet area of the gas channels 2 in theplate 1.

The flow resistance generation section 9 has, for example, such astructure as shown in FIG. 4. FIG. 4( a) is a plan view of the flowresistance generation section 9 and FIG. 4( b) is a front view of thesame. The flow resistance generation section 9 is formed by a thinplate-like base plate 9A having a connection section 9B, in whichprojection pieces 91 in the form of teeth are arranged with apredetermined spacing in one end of the base plate 9A. In this case, anozzle hole 92 is disposed in each projection piece 91 in such a waythat it passes through the center thereof from the other end of the baseplate 9A.

The flow resistance generation section 9 can be formed in a unified bodyfrom a material, which is selectable from synthetic resin, such aspolyacetal, polymethylpentene, polyphenylene ether, polyphenylen sulfideand liquid crystal polymer. Any resin for the material can be used, solong as it provides an excellent fluidity in the molding process, a highprecision in the finishing, an appropriate flexibility and an excellentthermal conductivity.

The flow resistance generation section 9 is designed, as for the size,to fit on the concave portion (not shown) in the inlet of the gaschannels 2, and as for the thickness, such that the upper surface of thesection 9 is located at the same level as the upper surface of the plate1, when it is fitted on the concave portion. The flow resistancegeneration section 9 is mounted onto the concave portion by adhesion. Inthis case, the mounting is carried out such that the projection pieces91 are inserted into the corresponding flow channels in the gas channels2. As a result, the gas inlet header 4 and the gas channels 2 aresecurely connected to each other via the nozzle holes 92. FIG. 3(a)shows that the gas inlet header 4 and the gas channels 2 are connectedto each other via the flow resistance generation section 9. The diameterof the nozzle hole 92 is about 0.25 mm on the side of the inlet (on theside of the gas inlet header 4), and 0.22 mm on the side of the outlet(on the side of the gas channels 2), and each nozzle hole 92 is taperedto some extent such that the gas passing through the hole can be ejectedtherefrom.

Referring to FIG. 3( a), FIG. 3( b), FIG. 3( c) and FIG. 3( d), asidefrom the water supply manifold hole 7, a second water supply manifoldhole 10 is disposed in both the plate 1 and the other plate 1′ so as topass therethrough. Referring to FIG. 3( b), cooling water is suppliedfrom the second water supply manifold hole 10, and in the water channels7B on the other surface (the second surface) of the plate 1, the coolingwater is introduced into an area located somewhat downstream from theflow resistance generation section 9 shown in FIG. 3( a). A differencebetween the second and the third embodiments also resides in such astructural arrangement.

Moreover, in FIG. 3( b), a second water discharge manifold hole 11 isdisposed on the upper left side of the plate 1 (on the side opposite tothe water supply manifold hole 7) so as to pass therethrough, and it isconnected to the water header 7D. In this case, the water supply header7D is separated from the inlet (the water inlet header 7A) of the waterchannels 7B for supplying the cooling water by disposing a partitionwall in the interface to the inlet of the water channels 7B. Adifference between the second and the third embodiments also resides insuch a structural arrangement.

In the third embodiment, as shown in FIG. 3( b), the cooling water issupplied from the second water supply manifold hole 10 to the waterchannels 7B in the plate 1, and the flows from top to bottom in thewater channels 7B. Thereafter, the cooling water is discharged from theoutlet of the water channels 7B to the water supply manifold hole 8, andit is further supplied from the water discharge manifold hole 8 to thewater supply manifold hole 7 as shown in FIG. 3( b). Moreover, thecooling water is supplied to the water supply header 7D and dischargedfrom the water supply header 7D to the second water discharge manifoldhole 11, and then flows in the stacking direction of the fuel cellstack, and finally discharged from the end portion of the fuel cellstack to the outside.

In the above water circulating channel, the means for supplying thecooling water from the water discharge manifold hole 8 to the watersupply manifold hole 7 can be realized, for example, by concavegroove-shaped channels (not shown) which are connected to the waterdischarge manifold hole 8 and to the water supply manifold hole 7 on theother surface of the plate 1, or by a tube-shaped connection channeldisposed either in the end plate of the fuel cell stack or outside thefuel cell stack such that the water discharge manifold hole 8 isconnected to the water supply manifold hole 7. In this case, the coolingwater is supplied in the water channels 7B in the plate 1 and thenreturned to the water supply header 7D in the plate 1.

The reason why the cooling water is supplied from the second watersupply manifold hole 10 is due to the fact that the polymer electrolytemembrane in the cell connecting to the gas channel 2 is humidified andmaintained in the saturated moist state, in which case, the coolingwater cools the inlet area for the water channels 7B, and further coolsthe inlet area for the gas channels 2 facing the water channels 7B onthe other surface, so that the dew point of the fuel gas is lowered whenthe fuel gas is introduced into the gas channels 2, and thereby thewater vapor contained in the fuel gas is compulsively condensed.

Furthermore, the reason why the cooling water passed through the waterchannels in the plate 1 is again returned to the water supply header 7Dis due to the fact that the area surrounded by the broken line in FIG.3( b) is warmed up, in which case, the flow resistance generationsection 9 is disposed in an area facing the above-mentioned area on theother surface, and the flow resistance generation section 9 is warmed upby the heat conduction, so that the dew condensation in the nozzle holes9 is prevented.

In the inlet area of the gas channels 2, the fuel gas is maintained inthe state where the dew condensation occurs easily, thereby making itpossible to prevent the polymer electrolyte membrane from being driedup. In this case, a fuel gas is ejected from the nozzle hole 92 of theflow resistance generation section 9, even if the water vapor isexcessively condensed in the gas channel 2. Therefore, the condensedwater adhered to the inner wall of the gas channel 2 is blown off, andit can be moved to the outlet at the downstream. As a result, the flowof the fuel gas is no longer hindered, because the condensed water doesnot clog the gas channels 2, and therefore a reduction in theperformance of the fuel cell can be prevented before it happens.

In the third embodiment where the flow resistance generation sections 9are equipped and the return of the cooling water is provided, it ispreferable that the following conditions are set up: The dew point ofthe air ≧the dew point of the fuel gas ≧the temperature at the inlet forthe cooling water in the second water supply manifold hole 10; and atthe same time, the dew point of the fuel gas ≦the temperature at theinlet for the cooling water in the water supply manifold hole 7.

In the following, referring to the accompanying drawings, theembodiments of a fuel cell system according to the invention will bedescribed.

FIG. 6 is a schematic sectional view of a component in a fuel cellstack. In FIG. 6, reference numeral 21 means a bipolar plate. On oneside of the bipolar plate, a concave groove-shaped fuel gas channel 21 ais in parallel disposed in the form of a straight channel, and on theother side of the bipolar plate a concave groove-shaped oxidant gaschannel 21 b is also in parallel disposed in the form of a straightchannel.

FIG. 7( a) is a plan view of a bipolar plate 21 viewed from the side ofthe fuel gas channels. A concave gas inlet header 21 c connected to eachfuel gas channel 21 a is disposed in the inlet of the fuel gas channel21 a, and a fuel gas supply manifold 21 d is connected to the gas inletheader 21 c. Similarly, a concave gas outlet header 21 e connected toeach fuel gas channels 21 a is disposed in the outlet of the fuel gaschannel 21 a, and a fuel gas discharge manifold 21 f is connected to thegas outlet header 21 e. Moreover, a nozzle-shaped flow resistancegeneration section 21 g is mounted in the inlet area of the fuel gaschannels 21 a, so that the cross section of each fuel gas channel 21 ais reduced. As a result, the fuel gas flows from the fuel gas supplymanifold 21 d to the gas inlet header 21 c and then flows into each fuelgas channel 21 a after accelerated by the flow resistance generationsection 21 g. Thereafter, the fuel gas is discharged from the outlet ofthe fuel gas channels 21 a to the gas outlet header 21 e before comingtogether, and finally discharged into the fuel gas discharge manifold 21f.

FIG. 7( b) is a plan view of the bipolar plate 21 viewed from the sideof the oxidant gas channels. A concave gas inlet header 21 h connectedto each oxidant gas channel 21 b is disposed in the inlet of the oxidantgas channels 21 b and a concave gas outlet header 21 i connected to eachoxidant gas channel 21 b is disposed in the outlet of the oxidant gaschannels 21 b. In the inlet area of the oxidant gas channels 21 b, anozzle-shaped flow resistance generation section 21 j is mounted, sothat the cross section of each oxidant gas channel 21 b is reduced. As aresult, the oxidant gas of air flows into the gas inlet header 21 h, andthen flows into each oxidant gas channel 21 b after accelerated by theflow resistance generation section 21 j. Thereafter, the oxidant gas isdischarged from the outlet for the oxidant gas channels 21 b to the gasoutlet header 21 i, and then discharged to the outside. In FIGS. 7( a)and 7(b), reference numerals 21 k and 21 m mean a water supply manifoldand water discharge manifold, respectively.

In FIG. 6, reference numeral 22 means an anode cooling plate. A concavegroove-shaped fuel gas channels 22 a are in parallel disposed in astraight groove on one surface of the anode cooling plate, and similarlya concave groove-shaped heat medium channels 22 b are in paralleldisposed in a straight groove on the other surface of the anode coolingplate.

FIG. 8( a) is a plan view of the anode cooling plate 22 viewed from theside of the fuel gas channels. A concave gas inlet header 22 c connectedto each fuel gas channel 22 a is disposed in the inlet of the fuel gaschannels 22 a, and the gas inlet header 22 c is connected to a fuel gassupply manifold 22 d. Similarly, a concave gas outlet header 22 econnected to each fuel gas channel 22 a is disposed in the outlet of thefuel gas channels 22 a, and the gas outlet header 22 e is connected to agas discharge manifold 22 f. Moreover, a nozzle-shaped flow resistancegeneration section 22 g is mounted in the inlet area of the fuel gaschannels 22 a, so that the cross section of each fuel gas channel 22 ais reduced. As a result, the fuel gas flows from the fuel gas supplymanifold 22 d to the gas inlet header 22 c, and then flows into eachfuel gas channel after accelerated by the flow resistance generationsection 22 g. Thereafter, the fuel gas is discharged from the outlet ofthe gas channels 22 a to the gas outlet header 22 e before comingtogether, and then discharged to the fuel gas discharge manifold 22 f.

FIG. 8( b) is a plan view of the anode cooling plate 22 viewed from theside of the heat medium channels. A concave heat medium inlet header 22h connected to each heat medium channel 22 b is disposed in the inletfor the heat medium channels 22 b, and the heat medium inlet header 22 his connected to a heat medium supply manifold 22 k. Similarly, a concaveheat medium outlet header 22 i connected to each heat medium channel 22b is disposed in the outlet for the heat medium channels 22 b, and theheat medium outlet header 22 i is connected to a heat discharge manifold22 m. As a result, the heat medium of water flows from the heat mediumsupply manifold 22 k to the heat medium inlet header 22 h, and thenflows into each heat medium channel 22 b. Thereafter, the heat medium isdischarged from the outlet of the heat medium channels 22 b to the heatmedium outlet header 22 i before coming together, and finally dischargedto the heat medium discharge manifold 22 m.

The anode cooling plate 22 thus structured is arranged such that thesurface of the anode cooling plate 22 on the side of the fuel gaschannels 22 a faces the surface on the side of the oxidant gas channels21 b of the bipolar plate 21, and that a cell (membrane electrodeassembly: MEA) is inserted between the above-mentioned surfaces. Then, agasket G is disposed in such a way that it surrounds the periphery ofthe cell.

In FIG. 6, reference numeral 23 means a cathode cooling plate. A concavegroove-shaped oxidant gas channels 23 b are disposed in a straight andparallel flow channel on one surface of the cathode cooling plate 23.

FIG. 9( a) is a plan view of the cathode cooling plate 23 viewed fromthe side of the oxidant gas channels 23 b. A concave gas inlet header 23h connected to each oxidant gas channel 23 b is disposed in the inletfor the oxidant gas channels 23 b, and a concave gas outlet header 23 iconnected to each oxidant gas channel 23 b is disposed in the outlet forthe oxidant gas channels 23 b. A nozzle-shaped flow resistancegeneration section 23 g is mounted to the inlet area for the oxidant gaschannels 23 b, and the cross section of each oxidant gas channel 23 b isreduced. As a result, the oxidant gas of air flows into the gas inletheader 23 h, and further flows into each oxidant gas channel 23 b afteraccelerated by the flow resistance generation section 23 g. Thereafter,the oxidant gas is discharged from the outlet for the oxidant gaschannels 23 b to the gas outlet header 23 i and finally discharged tothe outside. FIG. 9( b) is a plan view of the cathode cooling plate 23,viewed from the side on which the oxidant gas channels 23 b are notformed. In FIGS. 9( a) and 9(b), reference numerals 23 d, 23 f, 23 k and23 m mean a fuel gas supply manifold, fuel gas discharge manifold, heatmedium supply manifold and a heat medium discharge manifold,respectively.

The cathode cooling plate 23 is positioned such that the side on whichthe oxidant gas channels 23 b are not formed faces the side of the heatmedium channels 22 b in the anode cooling plate 22, as shown in FIG. 6.Namely, FIG. 6 shows that the oxidant gas inlet header 23 h which isshown in FIG. 9( a) is positioned to be back to back with the heatmedium inlet header 22 h or the heat medium outlet header 22 i. The heatmedium inlet header 22 h and the heat medium outlet header 22 i areshown in FIG. 8( b). Moreover, regarding the cathode cooling plate 23,the side on which the fuel gas channels 21 a in the bipolar plate 21having the same structure as in the above-mentioned bipolar plate 21 areformed faces the side of the oxidant gas channels 23 b, and the bipolarplate 21 is positioned by inserting a cell (membrane electrode assembly:MEA) therebetween. In this case, a gasket G is also mounted such that itsurrounds the periphery of the cell.

The respective plates are combined with each other in theabove-mentioned sequence and then stacked. Moreover, end plates (notshown) are attached to both ends of the plates thus stacked, and thenfastened by rods or the like to form a fuel cell stack. In therespective plates, the fuel gas supply manifold, fuel gas dischargemanifold, heat medium supply manifold and the heat discharge manifoldprovide through holes aligned in the stacking direction of the fuel cellstack. Thus, the fuel cell stack can also be constituted exclusively bythe combination of an anode cooling plate and a cathode cooling platewithout any bipolar plate.

FIG. 10 is a block diagram of an embodiment of a fuel cell systemaccording to the invention. In this case, an air humidifier 25 isconnected to a heat medium discharge outlet 24 a of a fuel cell 24, anda fuel humidifier 26 is connected to the air humidifier 25. Furthermore,a heat exchanger 27 is connected to the fuel humidifier 26. Thus, awater circulation channel 28 for the cooling water as the heat medium isconstituted by connecting the heat exchanger 27 to a heat medium supplyopening 24 b in the fuel cell 24.

Moreover, a total heat exchanger 29 is connected to an oxidant gasdischarge opening 24 c in the fuel cell 24 and the air humidifier 25 isconnected to the total heat exchanger 29. Thus, an air supply channel 30for the oxidant gas of air is constituted by connecting the airhumidifier 25 to an oxidant gas supply opening 24 d. Furthermore, anoxidant gas supply opening 24 d is disposed in an external manifold (notshown) mounted onto the upper part of the fuel cell stack for supplyingthe oxidant gas, and an oxidant gas discharge opening 24 c is disposedin an external manifold (not shown) mounted onto the lower part of thefuel cell stack for discharging the oxidant gas. Thus, the fuel cellsystem is constituted such that the oxidant gas of air discharged fromthe oxidant gas channels in the respective plates comes altogether, andis discharged from the oxidant gas discharge opening 24 c.

Moreover, a fuel reforming apparatus 31 is connected to the fuelhumidifier 26, so that the fuel reforming apparatus 31 converts a rawfuel such as town gas or the like to a reformed gas containing hydrogenas a main component. In the fuel cell system, the reformed gas ishumidified by the fuel humidifier 26 and then supplied to the fuel gassupply opening 24 e in the fuel cell 24. The humidification is carriedout by injecting the reformed gas into water stayed in the inside of thefuel humidifier 26. A wet fuel gas supplied to the fuel gas supplyopening 24 e in the fuel cell 24 is supplied into the connection holealigned in the stacking direction of the fuel cell stack by the fuel gassupply manifold, and then distributed fuel gas inlet headers in therespective plates, and further flows along the respective fuel gaschannels. The respective fuel gasses discharged from the fuel gaschannels (the fuel gas discharged without reaction) coming together inthe gas outlet section, and are discharged to the outside after passingthrough the connection hole in the stacking direction of the fuel cellstack. Generally, the fuel gas discharged to the outside withoutreaction is supplied from the fuel gas discharge opening 24 f to areforming burner in the fuel reforming apparatus and burned therein.

The air introduced as an oxidant gas from the outside exchanges the heatwith water by the total heat exchanger 29, and then it is supplied tothe oxidant gas supply opening 24 d (in a more detailed expression, theoxidant gas supply opening in the external manifold) of the fuel cell 24via the air humidifier 25. Water stays in the inside of the airhumidifier 25, and the air is humidified by injecting the air into thewater. A wet air supplied to the oxidant gas supply opening 24 d of thefuel cell 24 is distributed into the gas inlet headers in the respectiveplates and flows along the respective oxidant gas channels. The airsdischarged from the respective oxidant gas channels (the airs dischargedwithout reaction) comes together in the gas outlet header, and thendischarged from the oxidant gas discharge opening 24 c in the fuel cell24 (in a more detailed expression, the oxidant gas outlet in theexternal manifold). The un-reacted air thus discharged is furtherdischarged to the outside via the total heat exchanger 29.

As described above, the fuel gas and the oxidant gas are supplied to thefuel cell 24, so that an electrochemical reaction takes place via thesolid polymer electrolyte membrane in the cell (membrane electrodeassembly), thereby enabling a DC electric power to be generated. On theother hand, the water in the water circulating channel 28 is supplied tothe heat medium supply opening 24 b in the fuel cell 24, and flows theconnection hole in the stacking direction in the fuel cell stack, andthen distributed in the heat medium inlet headers of the respectiveanode cooling plates 22. The waters thus distributed flow along therespective heat medium channels, and the waters discharged from therespective heat medium channels come together in the heat medium outletheader. Thereafter, the water thus combined passes through theconnection hole aligned in the stacking direction of the cell stack, andfinally discharged from the heat medium discharge opening 24 a.

The anode cooling plate 22 is disposed such that the heat mediumchannels 22 b face the fuel gas channels 22 a on the other side, asdescribed above, thereby enabling the anode cooling plate 22 to becooled. Furthermore, the heat medium channels 22 b in the anode coolingplate 22 face the surface on which the oxidant gas channels of thecathode cooling plate 23 are not formed, thereby enabling the cathodecooling plate 23 to be cooled. As a result, the fuel cell 22 is cooledduring the operation period in the electric power generation, andthereby enabling the fuel cell 22 to be maintained at a proper operationtemperature (about 80° C.).

On the other hand, the heat medium of water discharged from the fuelcell 24 is heated up at a temperature of 78° C. or so. When water atsuch a high temperature is introduced into the air humidifier 25, thetemperature in the inside thereof can be increased. However, thetemperature of the water passed through the air humidifier 25 decreasesat 76° C. or so, and the water at such a medium temperature isintroduced into the fuel humidifier 26. The reformed gas at a hightemperature (100 to 150° C.) from the fuel reforming apparatus 31 isintroduced into the fuel humidifier 26, and injected into the water, asdescribe above, and therefore the water in the inside thereof ismaintained at 75 to 76° C. after losing the evaporation heat. The air issupplied to the air humidifier 25 after the dew point becomes 64° C. orso in the total heat exchanger 29.

The hot water passed through the fuel humidifier 26 is introduced intothe heat exchanger 27, in which the heat is exchanged between the hotwater and the water supplied from a water reservoir (not shown), andthen the water is returned to the water reservoir after changed into ahot water. The temperature of the water passed through the heatexchanger 27 is decreased to 74° C. or so. The water at such a lowtemperature is supplied to the heat medium supply opening 24 b in thefuel cell 24. Accordingly, the heat in the cooling water as a heatmedium can be efficiently used by circulating the cooling waterdischarged from the fuel cell 24 via the water circulating channel 28.

In accordance with the present invention, the dew point of the reactiongas is set at the temperature of the heat medium or smaller in the inletarea of the reaction gas, and the dew point of the reaction gas is setat the temperature of the heat medium or grater in the outlet area ofthe reaction gas.

Setting the temperature of the dew point of the reaction gas less thantemperature of the heat medium causes the reaction gas to be heated upin the inlet area by the heat medium, thereby making it possible toprevent the water vapor in the wet reaction gas from dew condensation inthe inlet area. Accordingly, the condensed water is not deposited ontothe gas channels in the inlet area of the reaction gas, so that thereaction gas starts to smoothly flow.

Setting the dew point of the reaction gas greater than the temperatureof the heat medium in the outlet area of the reaction gas causes thereaction gas to be cooled in the outlet area by the heat medium, therebythe water vapor in the reaction gas to be occasionally condensed.However, when the condensed water is adhered to the inner wall of thegas channels in the outlet area, a uniform pressure is applied to therespective gas channels, thereby enabling the water droplets to be blownaway. Accordingly, the condensed water can be discharged in a short timeto the gas outlet header. If the condensed water is adhered to the innerwall of part of the gas channels, and thereby clogs the gas channels, asin the prior art, uniform gas distribution does not occur in therespective gas channels, thereby causing an instable operation to takeplace in the power generation. In addition, the reaction gas isdeflected to the other gas channel, hence making it difficult to blowaway the water droplets. In accordance with the present invention, asdescribed above, the dew condensation is compulsively carried out insidethe respective gas channels in the outlet area of the reaction gas, sothat the pressure loss is homogenized over the gas channels, therebyenabling a uniform gas distribution to be attained.

In the above embodiments, it is described that the fuel gas flowsparallel to the oxidant gas from top to bottom in the direction ofgravity, whereas the heat medium flows in the direction anti-parallel tothe reaction gas. However, it is possible that the heat medium suppliesin the direction parallel to the reaction gas. In this case, it ispreferable that the cooling water discharged from the fuel cell 24 flowsin sequence from the heat recover heat exchanger 27, air humidifier 25,and the fuel humidifier 26, and then returned to the fuel cell 24.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made without departing from thespirit and scope of the invention. Accordingly, it is to be understoodthat the present invention has been described by way of examples, andnot by limitations.

1-17. (canceled)
 18. A fuel cell comprising a plurality of unit cellshaving been stacked on each other, each of the plurality of unit cellscomprising: a first cell having a first anode, a first cathode, and afirst electrolyte between the first anode and the first cathode; a firstfuel gas path, disposed between a first fuel gas inlet header and afirst fuel gas outlet header, for providing a fuel gas to the firstanode of the first cell; a first oxidant gas path, disposed between afirst oxidant gas inlet header and a first oxidant gas outlet header,for providing an oxidant gas to the first cathode of the first cell; afirst plate having a first surface and a second surface at opposite sideto the first surface, the first fuel gas path being between the firstsurface of the first plate and the first anode of the first cell; asecond plate having a third surface and a fourth surface at oppositeside to the third surface, the first oxidant gas path being between thethird surface of the second plate and the first cathode of the firstcell; a second cell having a second anode, a second cathode, and asecond electrolyte between the second anode and the second cathode; asecond fuel gas path, disposed between a second fuel gas inlet headerand a second fuel gas outlet header, for providing the fuel gas to thesecond anode of the second cell; a second oxidant gas path, disposedbetween a second oxidant gas inlet header and a second oxidant gasoutlet header, for providing the oxidant gas to the second cathode ofthe second cell; a third plate having a fifth surface and a sixthsurface at opposite side to the fifth surface, the second fuel gas pathbeing between the fifth surface of the third plate and the second anodeof the second cell; a fourth plate having a seventh surface and aneighth surface at opposite side to the seventh surface, the secondoxidant gas path being between the seventh surface of the fourth plateand the second cathode of the second cell; and a heat medium path,disposed between a heat medium inlet header and a heat medium outletheader, for distributing a heat medium, the heat medium path beingbetween the fourth surface of the second plate and the sixth surface ofthe third plate, wherein the heat medium inlet header or the heat mediumoutlet header is disposed to overlap with both the second fuel gas inletheader and the first oxidant gas inlet header in a perspective from adirection of the stacking the unit cells, wherein the fuel gas flowingthe second fuel gas path flows in the direction parallel to the oxidantgas flowing the first oxidant gas path, both the fuel gas flowing thesecond fuel gas path and oxidant gas flowing the first oxidant gas pathflow from top to bottom in the direction of gravity, and wherein theheat medium flowing the heat medium path between the fourth surface ofthe second plate and the sixth surface of the third plate flows in thedirection parallel or anti-parallel to the fuel gas flowing the secondfuel gas path and the oxidant gas flowing the first oxidant gas path andflows from top to bottom or from bottom to top in the direction ofgravity.
 19. The fuel cell according to claim 18, wherein the secondfuel gas path and the first oxidant gas path are substantially straight.20. The fuel cell according to claim 18, wherein the fuel cell furthercomprises a flow resistance generation section between the second fuelgas inlet header and the second fuel gas path and/or between the firstoxidant gas inlet header and the first oxidant gas path.
 21. The fuelcell according to claim 19, wherein the fuel cell further comprises aflow resistance generation section between the second fuel gas inletheader and the second fuel gas path and/or between the first oxidant gasinlet header and the first oxidant gas path.
 22. The fuel cell accordingto any one of claim 18 to claim 21, wherein a dew point of the fuel gasto be supplied to the second fuel gas inlet header and/or a dew point ofthe oxidant gas to be supplied to the first oxidant gas inlet headerless than or equal to a temperature of the heat medium to be supplied tothe heat medium inlet header in the case when the heat medium flowingthe heat medium path between the fourth surface of the second plate andthe sixth surface of the third plate flows in the direction parallel toboth the fuel gas flowing the second fuel gas path and the oxidant gasflowing the first oxidant gas path in normal operation, and wherein adew point of the fuel gas to be supplied to the second fuel gas inletheader and/or a dew point of the oxidant gas to be supplied to the firstoxidant gas inlet header less than or equal to a temperature of the heatmedium discharged from the heat medium outlet header in the case whenthe heat medium flowing the heat medium path between the fourth surfaceof the second plate and the sixth surface of the third plate flows inthe direction anti-parallel to both the fuel gas flowing the second fuelgas path and the oxidant gas flowing the first oxidant gas path innormal operation.
 23. The fuel cell according to claim 22, wherein a dewpoint of the fuel gas to be supplied to the second fuel gas inlet headeris less than or equal to a temperature of the oxidant gas to be suppliedto the first oxidant gas inlet header, or a temperature of the fuel gasto be supplied to the second fuel gas inlet header is greater than orequal to a dew point of the oxidant gas to be supplied to the firstoxidant gas inlet header in normal operation.
 24. The fuel cellaccording to claim 22, wherein a dew point of the fuel gas dischargedfrom the second fuel gas outlet header and/or a dew point of the oxidantgas discharged from the first oxidant gas outlet header greater than orequal to a temperature of the heat medium discharged from the heatmedium outlet header in the case when the heat medium flowing the heatmedium path between the fourth surface of the second plate and the sixthsurface of the third plate flows in the direction parallel to both thefuel gas flowing the second fuel gas path and the oxidant gas flowingthe first oxidant gas path in normal operation, and wherein a dew pointof the fuel gas discharged from the second fuel gas outlet header and/ora dew point of the oxidant gas discharged from the first oxidant gasoutlet header greater than or equal to a temperature of the heat mediumto be supplied to the heat medium inlet header in the case when the heatmedium flowing the heat medium path between the fourth surface of thesecond plate and the sixth surface of the third plate flows in thedirection anti-parallel to both the fuel gas flowing the second fuel gaspath and the oxidant gas flowing the first oxidant gas path in normaloperation.
 25. The fuel cell according to claim 23, wherein a dew pointof the fuel gas discharged from the second fuel gas outlet header and/ora dew point of the oxidant gas discharged from the first oxidant gasoutlet header greater than or equal to a temperature of the heat mediumdischarged from the heat medium outlet header in the case when the heatmedium flowing the heat medium path between the fourth surface of thesecond plate and the sixth surface of the third plate flows in thedirection parallel to both the fuel gas flowing the second fuel gas pathand the oxidant gas flowing the first oxidant gas path in normaloperation, and wherein a dew point of the fuel gas discharged from thesecond fuel gas outlet header and/or a dew point of the oxidant gasdischarged from the first oxidant gas outlet header greater than orequal to a temperature of the heat medium to be supplied to the heatmedium inlet header in the case when the heat medium flowing the heatmedium path between the fourth surface of the second plate and the sixthsurface of the third plate flows in the direction anti-parallel to boththe fuel gas flowing the second fuel gas path and the oxidant gasflowing the first oxidant gas path in normal operation.